The present invention relates to data communications and more particularly to systems and methods for improving the performance of point to multipoint networks.
A point to multipoint wireless communication system represents a potentially effective solution to the problem of providing broadband network connectivity to a large number of geographically distributed points. Unlike optical fiber, DSL and cable modems, there is no need to either construct a new wired infrastructure or substantially modify a wired infrastructure that has been constructed for a different purpose. In a typical wireless point to multipoint network design, there is a central access point that provides connectivity to the Internet backbone and numerous subscriber units that interact directly with the central access point. Communication from the head end or central access point to the individual subscriber units is referred to as downstream communication. Communication from a subscriber unit to the central access point is referred to as upstream communication.
In one scenario, upstream and downstream communications use different frequencies. A time division multiple access (TDMA) scheme may be used to divide access to the upstream communication channel among the various subscriber units.
Except for the physical medium, the wireless point to multipoint network architecture just described is in fact very similar to the architecture of a cable modem network as defined by the DOCSIS standard promulgated by Cable Television Laboratories, Inc. In order to take advantage of the large body of experience in operating cable modem networks and further to be able to employ components already developed for cable modem networks, it is desirable to adapt at least the media access control (MAC) layer portion of the DOCSIS protocol to the wireless point to multipoint network while employing a physical layer that is suitable for wireless applications. The physical wireless communication channel generally represents a greater challenge than the cable channel due to noise, interference and greater multipath effects. Adapting DOCSIS to the wireless environment involves in part using a more robust physical layer such as one based on OFDM (orthogonal frequency division multiplexing).
Network protocols designed from the start for wireless communication often include a mechanism referred to as “ARQ” (Automatic Repeat Request) that provides for acknowledgment of successfully transmitted information and retransmission where there is no such acknowledgment. ARQ increases the performance of wireless communication networks in challenging channel conditions because the odds of successive transmission are greatly increased if multiple attempts are permitted. Unfortunately, DOCSIS makes no provision for ARQ in its MAC layer design. Furthermore, it is very cumbersome to include ARQ at the physical layer and still take advantage of DOCSIS MAC layer components.
What is needed are systems and methods for adapting ARQ to the DOCSIS MAC layer while making minimal changes to the DOCSIS protocol. It is also desirable to minimize overhead and capacity reduction associated with requesting retransmission and retransmitting.